Method for the determination of acid concentrations



Sept. 15, 1970 J, M KAVENEY ET AL 3,528,778

METHOD FOR THE DETERMINATION OF ACID CONCENTRATIONS Filed June 27, 1968d n a Y M m n m A M. .M P. 8 0 E M v A /v 5 .2 J Km M \\fl\v I: a u 66 vff A L. w 4 v m mn n m VEN roRs.

CHARLES .1. am/vzs M Alrbr ney United States Patent U.S. CI. 23-230 11Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for determiningthe concentration of acid present in acidic aqueous solutions, such asindustrial pickling baths and other metal-treating baths. The inventionis performed by mixing a known quantity of an acidic aqueous solution,the concentration of which is to be determined with a fluoride in ionicform permitting the release of the fluoride ion in the mixture. Thefluoride is introduced in at least a stoichiometric amount with respectto the acid so that there are suflicient fluoride ions available forreaction with the available hydrogen ions of the acid to formhydrofluoric acid. This mixture is placed in an electrolytic cell havinga first electrode, which comprises the anode, of a semiconductingmaterial, such as 'n-type silicon material. A second electrode, which isthe cathode is additionally provided and is constructed of a conductingmaterial substantially resistant to chemical attack by the mixture, suchas stainless steel, platinum or titanium. An electrical potential isimpressed between the electrodes, which results in the production of alimiting electrical current between the electrodes. The magnitude ofelectrical cur rent has been found to be substantially, linearlyproportional to the magnitude of the acid concentration in the acidicaqueous solution.

This is a continuation-in-part of our copending application, Ser. No.649,211 filed on June 27, 1967, and now abandoned.

Until the present invention, acid measurement has depended in large parton either direct chemical analysis (titration with standard alkali) orindirect physical methods (solution electrical conductivity coupled withspecific gravity or metal ion measurement). Neither the chemical nor thephysical method is completely satisfactory for determining the acidconcentration in acidic aqueous solutions that are characterized by thepresence of metals, such as iron, chromium, neckel, manganese, titaniumand the like, in the solution, because the accuracy of theseconventional methods is impaired by the presence of such metals in thesolution being tested.

It is accordingly an object of the present invention to provide a methodand apparatus for determining the acid concentration of acidic aqueoussolutions that is not influenced with respect to the accuracy of thedetermination by concentrations of metals in the solution being tested.

This and other objects, as well as a complete understanding of theinvention, may be obtained from the following description and drawingsin which the single figure thereof shows an electrolytic cell andassociated electrical circuitry suitable for use in the practice of themethod of the invention.

In the practice of the invention, a known quantity of an acidic aqueoussolution, the acid concentration of which is to be determied, is mixedwith a fluoride in ionic form to provide an electrolyte. The electrolytemixture is placed in an electrolytic cell of the type designatedgenerally as 10 in the figure. The electrolytic cell 10 consists of anelectrolyte container, such as a standard beaker, designated as 12,within which the electrolyte mixture 13, as described above, iscontained. Immersed within the electrolyte of the container is anelectrode assembly consisting of a rod-shaped anode 14 of asemiconducting material surrounded by a cylindrical cathode 15. In thismanner, the cathode shields the anode to prevent damage thereto. Thecathode 15 is constructed from stainless steel or other conductingmaterials suitable to resist chemical attack by the elecrolyte. Both ofthe electrodes 14- and 15 are secured at their upper ends to apolyvinylchloride cap, designated as 16. The anode 14 is connected tocap 16 by a polyvinylchloride nut 16a, which is threaded into a tappedopening in the bottom of cap 16. The end of the anode 14 is embeddedwithin the nut 16a. This arrangement is provided to permit easyreplacement of the anode Without necessarily requiring replacement ofthe cathode and vice versa. The anode 14 is connected to the positivepole of a battery 17, and the cathode 15 is connected to the negativepole of the battery. Between the battery 17 and the anode 14, there iselectrically connected a conventional microammeter 18.

An oxidizing agent may be included in the electrolyte as describedabove, to permit increased etching of the anode or more uniform currentresponse and thus insure increased and uniform meter readings. Althoughit is possible to use any oxidizing agent that will permit etching ofthe anode in the presence of hydrofluoric acid, oxidizing agents thathave been found suitable for the purpose are hydrogen peroxide,potassium permanganate, potassium dichromate, bromine and ammoniumpersulfate. The fluoride is introduced to the electrolyte preferably inthe form of a soluble, non-complexed fluoride salt, such as potassiumfluoride, sodium fluoride and ammonium fluoride. The oxidizing agent andfluoride may be used in a ratio of, for example, 1 to 3. The fluorideion combines with the hydrogen ion of the acid in the electrolytesolution to form hydrofluoric acid. Since the fluoride is added in atleast a stoichiometric amount with respect to the acid, there aresufiicient fluoride ions available for reaction with the availablehydrogen ions of the acid. The quantity of available hydrogen ions is,of course, dependent upon the concentration of the acid in theelectrolyte. With an anode of semiconducting materials, such as n-typesilicon and n-type germanium material, the electrical current producedbetween the anode and cathode will be substantially, linearlyporportional to the amount of hydrofluoric acid produced in theelectrolyte, which is in turn substantially, linearly proportional tothe initial acid concentration of the electrolyte. By making calibrationdeterminations with a particular electrolytic cell with electrolytes ofknow varying acid concentration, as will be shown and. explained indetail hereinafter, it is possible to calibrate the particularelectrolytic cell and associated circuitry so that a specificlimiting-current magnitude will be known to correspond to a specificamount of acid concentration.

With the method as described above, it has been possible to accuratelymeasure acid concentrations of sulfuric acid, nitric acid, hydrochloricacid, phosphoric acid, acetic acid, sulfurous acid and perchloric acid.In addition to the above-mentioned acids, the invention would beapplicable to other inorganic and organic acids capable of ionization inaqueous solution. Such determinations are not possible in the absence ofthe above-described reaction wherein hydrofluoric acid is formed. Inaddition, if an oxidizing agent is used, the acid cannot react with theoxidizing agent in the electrolyte.

As a specific example of the practice of the invention a 1.00-ml. acidsample was transferred to a ZOO-ml. volumetric flask containing ml. ofdistilled water. To this 3 was added 10 ml. of a mixture containing 1.6g. of ammonium fluoride and 0.6 g. of ammonium persulfate. This wasdiluted to 200 ml. with water and mixed to provide the electrolyte. Theelectrolyte was used in an electrolytic cell of the type shown in thedrawing and described above. The cathode of the cell was constructedfrom stainless steel, and the anode was constructed from n-type siliconmaterial having a resistivity of 0.05 ohmcm. with a surface area of 4.7sq. cm. exposed to the electrolyte. The electrolyte remained in contactin the cell with the electrodes for about four minutes while a voltageof about 1.35 volts was impressed between the electrodes from a batterysource. The limiting current was recorded on a microammeter. The acidconcentration of the acid sample used in the electrolyte was varied foreach test from substantially concentration to a selected maximum acidconcentration. From the data so obtained and recorded, a relationship ofmicroamperes per gram of acid was obtained. This relationship over theconcentration range investigated is presented in Table I for variousacids subjected to testing. It may be seen from Table I that for a givencurrent, as expressed in microamperes, it is possible to immediatelydetermine the acid concentration, as expressed in grams per 100 ml. Forexample, if a current of 198 microamperes was obtained for a sulfuricacid sample, the concentration of sulfuric acid would be 2 gs. per 100ml. This is obtained by dividing the current of 198 microamperes by 99,as listed in Table I.

TABLE I Acid concentration range Microamps/fgram As another specificexample of the practice of the invention, a 10.0-ml. aliquot of acidsample was transferred to a 200-ml. volumetric flask containing 100 ml.of water. To this was added ml. of a solution containing 2.2 grams ofammonium fluoride. This was diluted to 200 ml. to provide theelectrolyte. The electrolyte was used in an electrolytic cell of thetype shown in the drawing and described above. The cathode of the cellwas constructed of stainless steel and the anode was constructed fromn-type silicon material having a resistivity of 0.01 ohm-cm. with asurface area of 4.5 sq. cm. exposed to the electrolyte. The electrolyteremained in contact in the cell with the electrodes for about fourminutes while a voltage of about 1.35 volts was impressed between theelectrodes from a battery source. The limiting current was recorded on amicroammeter. The acid concentration of the acid sample used in theelectrolyte was varied for each test from substantially 0 concentrationto a selected maximum concentration. From the data so obtained andrecorded, a relationship of microamperes per gram of acid was obtained.This relationship over the concentration range investigated is presentedin Table II for sulfurous acid.

TABLE II Acid concentration range Microamps/grain Acid (grains/100 ml.)of acid Sulturous (H2803) 0. 0-2. 8 405 of the invention will havevarying limiting-current characteristics depending upon factors such astotal anode surface area exposed to the electrolyte, temperature of theelectrolyte and anode material. As described above, the cell may bereadily calibrated by performing a series of tests with acid samples ofknown varying concentrations. Manual or automatic temperaturecompensation may be used to provide consistent acid measurements.

Although the invention is particularly adapted for use in determiningthe acid concentrations of steel pickling baths, it is also capable ofmany other uses. For example, by using relatively larger volume samplesand relatively less water dilution, the method would be applicable tovery dilute acid solutions, such as metal-rinse waters and natural orother waters.'This would render the invention extremely useful inanapplication such as stream-pollution monitoring; with the use of theinvention this could conceivably be achieved on a substantiallycontinuous basis.

Examples of suitable materials for the construction of the anode andtechniques for the production thereof are well known in the art. Suchare described, for example, in Handbook of Semiconductor Electronics,published in 1956 by McGraw-Hill.

The term limiting current as used herein refers to the current that isproduced between electrodes in the presence of a given minimum electrodepotential and a conducting electrolyte.

The term measuring of the limiting current, as used herein, embraces thewellknown operations of measuring current (amperes), as with an ammeter,measuring voltage (volts), as with a volt meter or measuring resistance(ohms), as with an ohmmeter.

What is claimed is:

1. A method for determining the concentration of an acid capable ofionization in water solution comprising mixing a known quantity of anacid-water solution with a fluoride in ionic form, said fluoride beingintroduced in at least a stoichiometric amount with respect to saidacid, immersing within said resulting mixture a first electrode of asemiconducting material from which a limiting current can be producedwhile immersed in said mixture, additionally providing a secondelectrode in electrical contact with said mixture, impressing anelectrical potential between said electrodes to produce a limitingelectrical current proportional to the acid concentration in saidmixture, and measuring said limiting electrical current to determine theacid concentration of said mixture.

2. The method of claim 1, wherein said first electrode material isn-type silicon.

3. The method of claim 1, wherein an oxidizing agent is included in saidmixture.

4. The method of claim 3, wherein said oxidizing agent is selected froma group consisting of hydrogen peroxide, potassium permanganate,potassium dichromate, bromine and ammonium persulfate.

5. The method of claim 1, wherein said fluoride in noncomplexed, ionicform is selected from the group consisting of potassium fluoride, sodiumfluoride and ammonium fluoride.

6. The method of claim 1, wherein said acid is selected from the groupconsisting of sulfuric acid, nitric acid, hydrochloric acid, phosphoricacid, acetic acid, sulfurous acid and perchloric acid.

7. A method for determining the concentration of an acid capable ofionization in water solution comprising mixing a known quantity of anacid-water solution selected from a group consisting of sulfuric acid,nitric acid, hydrochloric acid, phosphoric acid, acetic acid, sulfurousacid and perchloric acid, with an oxidizing agent selected from thegroup consisting of hydrogen peroxide, potassium permanganate, potassiumdichromate, bromine and ammonium persulfate, and a fluoride in ionicform selected from the group consisting of potassium fluoride,

sodium fluoride and ammonium fluoride, said fluoride being present in atleast a stoichiometric amount with re spect to said acid, immersingwithin said resulting mixture an anode of n-type silicon material,additionally immersing a cathode of a material resistant to chemicalattack by said mixture, impressing an electrical potential between saidanode and cathode to produce a limiting electrical current proportionalto the acid concentration within said mixture, and measuring saidlimiting electrical current to determine the acid concentration of saidmixture.

*8. An apparatus for determining the concentration of an acid capable ofionization in water solution comprising (1) a first electrode of asemiconducting material, (2) a second electrode, (3) an electrolyte foreffecting electrical contact between said first and second electrode,said electrolyte being a mixture of a known quantity of an acidwatersolution and a fluoride in ionic form with said fluoride being presentin at least a stoichiometric amount with respect to said acid, and (4)means for impressing an electrical potential between said electrodes toproduce a limiting electrical current proportional to the acidconcentration in said mixture, and means for measuring said limitingelectrical current to determine the acid concentration of said mixture.

9. The apparatus of claim 8, wherein said first electrode is constructedfrom n-type silicon material.

10. The apparatus of claim 8, wherein said first electrode is housedwithin said second electrode.

11. The apparatus of claim 10, wherein said second electrode is ofgenerally cylindrical construction.

References Cited UNITED STATES PATENTS 2,766,442 10/1956 Meyer 324-OTHER REFERENCES Turner, D. R.: Anal. Chem. 33%, No. 7, June 1961, pp.

MORRIS O WOLK, Primary Examiner R. M. REESE, Assistant Examiner US. Cl.X.R. 23-253; 20'4l,

